Method and apparatus for storing and reading out charge in an insulating layer

ABSTRACT

A diode storage array, including a diode array on one face of a semiconductor wafer, an insulating layer overlying the opposite face of said wafer and a conductive layer overlying the insulating layer, is written upon by irradiating the conductive layer side of said wafer to induce charge storage in the insulating layer. The radiation may be high energy photons, a scanned electron beam or electrons from a photo-emitter. Readout is accomplished by irradiating the target with lower energy radiation to form electron-hole pairs in the wafer. The holes are selectively driven to the diode side of the wafer under the control of the stored charge where selected, reverse biased, diodes are discharged. Subsequent scanning of the diode array by an electron beam produces a variable output signal, indicative of the information stored. Since the charge on the insulating layer is not dissipated, the information can be read as often as desired.

United States Patent Wllson et al.

Oct. 2, 1973 METHOD AND APPARATUS FOR STORING AND READING OUT CHARGE INAN lNSULATlNG LAYER [75] Inventors: Ronald H. Wilson; George E. Possin,

both of Schenectady; George W.

Ellis, Burnt Hills, all of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Mar. 15, 1972 [21] Appl. No.1 234,786

[52] [1.8. CI. 340/173 LS, 313/65 AB, 317/235 NA,

[51] Int. Cl. Gllc 5/02, G1 1c 11/34, Gllc 13/04 [58] Field of Search340/173 LS, 173 CR;

317/235 NA; 313/65 A, 65 T, 89, 65 AB [56] References Cited UNITEDSTATES PATENTS 3,701,979 10/1972 Smith 340/173 R 3,668,473 6/1972Miyashiro 317/235 NA 3,676,715 7/1972 Brojdo 340/173 LS 3,576,392 4/1971l-lofstein..... 317/235 NA Crowell......

Primary ExaminerBernard Konick Assistant Examiner-Stuart HeckerAttorneyJohn F. Ahern et al.

[57] ABSTRACT A diode storage array, including a diode array on one faceof a semiconductor wafer, an insulating layer overlying the oppositeface of said wafer and a conductive layer overlying the insulatinglayer, is written upon by irradiating the conductive layer side of saidwafer to induce charge storage in the insulating layer. The radiationmay be high energy photons, a scanned electron beam or electrons from aphoto-emltterfR'ad o'ut is'ac complished by irradiating the target withlower energy radiation to form electron-hole pairs in the wafer. Theholes are selectively driven to the diode side of the wafer under thecontrol of the stored charge where selected, reverse biased, diodes aredischarged. Subsequent scanning of the diode array by an electron beamproduces a variable output signal, indicative of the information stored.Since the charge on the insulating layer is not dissipated, theinformation can be read as often as desired. I

10 Claims, 1 Drawing Figure WRITE Patented Oct. 2, 1973 ERASE WRITE I I61 WM- 24 1 2 METHOD AND APPARATUS FOR STORING AND Another object of thepresent invention is to provide READING OUT CHARGE IN AN INSULATING adiode array storage target in which the reading and LAYER writingoperations can be performed simultaneously.

The foregoing objects are achieved in the present in- ThiS inventionelates to diode array storage targets 5 vention wherein information isstored as an electric and, in-particular, to a method for writing andreading charge in an insulating layer overlying one Surface 0f 3 with htargets, semiconductor wafer. Reading is accomplished by in- In theprior art, diode array targets are used f a ducing electron-hole pairsin the semiconductor and riety f f ti such as imaging devices and scanallowing the holes to diffuse through the wafer to the converters. Insuch applications attempts at providing opposite suffaceawhich Containsthe p-n junction array. storage capability in excess of the typical 1/30second, At the llmfmon the holes recombme the approximate time neededfor scanning the entire electronsPrevlousl'y deposltefi by an electronbeam to target once, have resulted in storage times on the reverse biasthe diodes. During a subsequent scan of order of a few seconds. Longerstorage times have been the array.by the electron vananons m curobtainedby mechanisms that, in effect, interfere with rent F i {estqre thedlqdes to a.reverse blased the operation of the target, thereby Slowingthe condition are indicative of the information stored.

. Since the charge is stored in the insulator it is not s ons of t p 6he arget For example a msulatmg dissipated. By inducing theelectron-hole pairs from an layer has been interposed between the diodearray and th 1 t b t d h ff f h external source, the information can beread as often e e cc 0 re uce rec argmg e o t e as desired orcontinuously. The information stored can electron beam during readout,thereby retaining some be erased or chan ed as desired b suitablirradiatin charge indicative of the information stored. However, g y y gthe insulating layer with the more energetic radiation. Storage tfirgatsof thls type that have an appreciable A more complete understanding ofthe present invenstorage time generally require a longer time forreadtion can be obtained by considering the following detaileddescription in conjunction with the accompany- Alternatively, it hasbeen proposed to store charge on ing drawings, in which;

the oxide on the diode side of the target. This, however, T FIGURE ill adi d array target system can compromise the operating characteristics ofthe i bl f use i h present invention g -g-. Charge P the oxide layerduring The FIGURE illustrates a preferred embodiment of non-storingoperation. Some diode array targets utilize the present inventionwherein a diode array camera one or more protective layers to preventcharge buildtube target is modified by the application of charge up onthe oxide. However, to provide storage capabilstorage layers to thesubstrate on the opposite side of ity, these protective layers cannot beused. the diode array. The net result is a combination that In addition,the information stored is destructively provides a unique cooperationbetween the charge read out in many of the systems of the prior art,i.e., the stored and diode array elements so as to provide a storchargerepresenting the information stored is utilized g Camera tube targetthat is x r m ly fle b e n 0pdirectly in the read operation and ispartially or totally eratiofldissipated. Further, many systems of theprior art are l? y target m comprises subtfate 11 not capable ofcarrying out the read and write functions Pnsmg an yp Semlconductormaterial Onto which simultaneously p-type conductivity regions 12 aregrown through a Thus, there is a need in the art f a method of writplurality of apertures in apertured insulating layer 13 ing and readingin diode array targets that provides collpled to Substrate is long term,i.e., virtually indefinite, erasable storage across Much the vldfaooutpu} Slgna] while also providing non-destructive readout,simultatamed1connected Feslstor 20 a l of neous read and writecapability, and read times compa potential 19 for suitably biasing thetarget relative to thode l7. rable to non-storing targets. ca

In View of the foregoing it is therefore an object of The target asthusdescribed is similar to a target described by William E. Engeler inapplication Ser. No. invention to pmvlde a method. for f g 60,767, filedAug. 3, 1970 and assigned to the same asinformation in a diode arraymemory so that the inforras bl b t a b tored for [on s de signee as thepresent invention. While this particular IS 6 a e u c n e s as g a formof diode array is shown and described it should be h f h t understoodthat any suitable diode array target may be not er 0 f? t e presen l toprov e utilized in carrying out the present invention. However, a methodfor writing and reading diode array targets d d this particular diodearray does not require the protecproducing long term storage andnonestructive rea five layers noted above out.

The face of substrate 11 opposite the face in which A further ob ect ofthe present invention lS.tO provide Hype conductivity regions 12 areformed is covered long Storage and rapld readout of dlode array byinsulating layer 14, which may conveniently commemol'les- 6o prise anoxide of substrate 1].. Layer M in turn is cov- Another object of the Pinvention is to Provide ered by a transparent conductive layer 15. Layer15 is a writing and reading method for diode array Storage connected tosources of operating potential 24 and 25 targets in which theinformation stored is not directly b way of potentiometer 16, fo lsources of utilized in reading. operating potential 24 and 25 arefurther connected to A further object of the present invention is toprovide a mm n oint comprising the junction of potential a writing andreading method for diode array storage source 19 and output resistor 20.Cathode 21 is biased targets in which the electrical effect of chargereprerealtive to the target by way of bias means 23 which is sentingstored information is utilized for reading. also connected to the commonpoint. Also illustrated in the FIGURE, is a source of light 26 producingphoton energy illustrated by rays 27.

The overall operation of the present invention shall be described inthree parts. The first relating to the diode array, left-hand portion ofthe target illustrated in the FIGURE, the second relating to theoperation of V the right-hand portion of the target as illustrated inthe FIGURE, and finally the cooperation of these two sections together.

Ignoring for the moment the presence of layers 14 and 15, the operationof a diode array target is relatively well known and may be summarizedas follows: As an initial step, the diode array is scanned by electrons18 eminating from cathode 17. This scan negatively charges p-typeconductivity regions 12 relative to substrate 11. Input information,generally an optical image, applied to the opposite face of thesubstrate 11 forms a pattern of electron hole pairs, the holes of whichdiffuse to the diodes and discharge the diodes in proportion to theintensity of the light absorbed in that area of the substrate. As theelectron beam rescans the diode side of the substrate, the currentnecessary to recharge the p-type conductivity regions is proportional tothe amount by which the p-type conductivity regions were discharged.This current flows in a circuit comprising cathode 17, electron beam 18,target and output resistor 20. Thus, the current flowing through outputresistor 20 provides a video signal corresponding to the pattern oflight incident upon the substrate. It may be noted that the operation ofthis type of target requires the continuous application of inputinformation on the opposite face of the substrate. The storage time ofthe diode portion of the target is dependent upon the time it takes thedark current to completely discharge the diodes.

The right-hand side of target 10 as illustrated in the FIGURE operatesto store charge in proportion to input information in insulating layer14 for relatively long periods of time, e.g., several tens of hours. Thestorage of charge in insulator 14 is not permanent, i.e., theinformation can be readily erased.

The charge in insulating layer 14 can be created by a variety of means,such as a scanned electron beam, electrons from a photoemitter, or highenergy photons. For the preferred embodiment of the present invention,it will be assumed that the pattern of charge is to be obtained from anelectron beam.

Electron beam 22 emanating from cathode 21 is directed so as to writethe information in a predetermined pattern as charge in insulating layer14. This is accomplished by utilizing a high energy electron beam, forexample, 10 kilovolts. During the writing operation, transparentconductive layer is biased positively with respect to n-typeconductivity substrate 11. In so doing, mobile electrons induced ininsulating layer 14 by electron beam 22 are drawn off throughtransparent conductive layer 15. Electron beam 22 is then terminated anda pattern of positive charges 30 is stored in insulating layer 14.

The charge stored in insulating layer 14 can be removed in the same wayit was created, except that transparent conductive layer 15 is biasednegatively or not at all with respect to substrate 11. Thus, thechangeover from one mode of operation to another is very simply andeasily accomplished. After all or part of the information is erased, asubsequent writing operation is performed to store new information inthe erased areas.

The pattern of positive charges 30 is retained by insulating layer 14for a relatively long time. As will be apparent from the followingdescription of the operation of the two halves of the target together,the charge stored in insulating layer 14 is not used directly in theread out of the pattern of charged storage. Further, it should be notedthat the writing operation and the reading operation are independent sothat no restriction is placed on the operation of either half of target10.

During readout, the storage face of target 10 is illuminated by lightfrom source 26. Since the energy contained in a photon is proportionalto the frequency (v) of the photon it is preferable to illuminate thetarget with light of frequency in the range of visible light so that thepositive charge pattern 30 is not changed during the reading operation,regardless of the setting of potentiometer 16.

The incident photons penetrate into n-type conductivity substrate 11thereby forming electron-hole pairs. Due to the storage of positivecharge, the holes of the electron-hole pairs are repelled away from thestorage side of target 10 to the diode side of target 10 where theydischarge those diodes approximately opposite the location of the storedpositive charge. In the absence of stored charge the electron-hole pairsdiffuse t0 the interface between substrate 11 and insulator 14 wherethey recombine and are lost. For a silicon wafer, blue light ispreferred so that the holes are created near the storage layer therebyincreasing the modulation efficiency of the stored charge pattern. Thus,the pattern of charge storage is transferred from the right-hand side oftarget 10 as illustrated in the FIGURE to the left-hand side of target10. Readout of the diode array is then accomplished as described above,wherein the diodes are scanned by an electron beam and the amount ofcharge necessary to restore the charged condition is monitored acrossoutput resistor 20.

As noted above, the readout and charge storage operations are completelyindependent since the stored charge is only used indirectly indischarging the diodes. This enables what may be generally described asspecial effects to be performed with the stored information. Forexample, the storage of information and the scanning by electron beam 18can occur at different rates. Also, the pattern of scanning need not bethe same for both the storage of information and the reading ofinformation. For example, apparent motion can be obtained in an image bymodifying the location of the starting point of the scan by electronbeam 18.

It should be emphasized that radiation in the visible region incidentupon the storage side of target 10 has no effect on the stored chargeregardless of the setting of potentiometer 16. Information is writtenonly with higher energy radiation when transparent conductive layer 15is biased positively relative to substrate 11. Erasure of the entirestored charge is carried out by negatively biasing transparentconductive layer 15 during higher energy irradiation, as by electronbeam 22. In this case, mobile electrons are induced and recombine withthe positive charge stored in layer 14 to bring the net charge oninsulating layer 14 to zero. Obviously, erasure can be either partial(one area or only part of the charge) or total (all of layer 14 or allof the charge). It should be further noted that no structure is requiredon the storage side of target 10, thereby enabling a maximum ofresolution to be obtained. The storage side of target operates bycontrolling the recombination of induced electron-hole pairs byrepelling the holes from the storage side of the target so that theydiffuse to the diode side of the target where they discharge the diodesin the same pattern as the stored charge.

Target 10 can be fabricated in any suitable fashion, as, for example,set forth in the above-noted application of William E. Engeler, with theaddition of layers 14 and 115. Substrate 11 can be a silicon wafer onthe order of p. thick and the apertures through which ptype conductivityregions 12 are grown can be on the order of 8 p. in diameter. Insulatinglayer 14, which may comprise several layers of insulating material, canbe on the order of 5,000A. thick, necessitating a write/erase beam inexcess of about SkV. The intensity of the illumination from blue lightsource 26 can be on the order of (10) watts per square cm. The biasvoltage on transparent conductive layer 15 can be fiO volts. It is to beunderstood that the foregoing values are exemplary only and notlimiting.

Having thus described the invention it will be apparent to those skilledin the art that various modifications can be made without departing fromthe spirit and scope of the present invention. For example, aspreviously noted, p-type conductivity regions 12 need not be epitaxiallygrown but may be formed simply as diffused regions. The source of biaspotential for transparent conductive layer need not be as shown but mayemploy other, more elaborate sources, such as pulses for selectingwrite, neutral and erase in synchronism with the deflection of beam 22.Also, any suitable deflecting means may be employed.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An information storage target comprising:

a substrate of a first type conductivity semiconductor having first andsecond opposite sides;

a diode array formed on said first side;

an insulating layer, overlying said second side, for

storing information in the form of electric charge; and

a conductive layer, overlying said insulating layer, for

controlling the writing and erasing of information in said insulatinglayer.

2. The method of writing, storing, reading and erasing information in adiode array target, containing a diode array on one face of asemiconductor wafer, an insulating layer overlying the opposite face ofsaid wa fer, and a transparent, conductive layer overlying theinsulating layer, comprising the steps of:

selectively irradiating the insulating layer to induce mobile electronstherein;

drawing off said mobile electrons to selectively charge said insulatinglayer by biasing said conductive layer to one polarity;

inducing electron-hole pairs in said wafer;

preventing recombination of said pairs and allowing at least one of saidelectrons and holes to diffuse to the diode side of said wafer; and

5 scanning the diode side of said wafer with an electron beam to readout said stored information.

3. The method as set forth in claim 2 and further comprising the stepsof:

biasing said conductive layer to a second, opposite 10 polarity; and

irradiating the insulating layer to induce mobile electrons in saidinsulator for neutralizing the charge on said insulating layer. 4. Themethod as set forth in claim 2 wherein said inl 5 ducing step comprises:

irradiating said target with photons of light.

5. The method as set forth in claim 2 wherein the step of selectivelyirradiating comprises:

selectively irradiating the insulating layer with an electron beampenetrating said conductive layer to induce mobile electrons in saidinsulating layer.

6. An information storage system comprising a semiconductor diode arraytarget comprising a planar semiconductor substrate having a diode arrayon one side and an insulating layer covering the opposite side;

means for storing information in the form of electric charge in saidinsulating layer; means for irradiating said opposite side of saidtarget for inducing electron-hole pairs in said substrate, said storedcharge preventing recombination of said pairs to allow diffusion of atleast one of said electrons and holes toward the diode array side ofsaid target, thereby transferring the information from one side of thetarget to the other; and

means for reading said information from said diode array.

7. The information storage system as recited in claim 6 wherein saidtarget further comprises a transparent, conductive layer overlying saidinsulating layer and said means for storing information comprises:

a source of electron beam for irradiating selected portions of saidinsulating layer; and

bias means coupled to said transparent, conductive layer and saidsubstrate for biasing said conductive layer relative to said substrate.

8. The information storage system as set forth in claim 6 wherein saidmeans for reading said information comprises:

a source of an electron beam for irradiating selected portions of saiddiode array.

9. The information storage system as set forth in claim 6 wherein saidmeans for irradiating said target comprises a source of light.

10. The information storage system as set forth in claim 9 wherein saidlight is blue.

1. An information storage target comprising: a substrate of a first typeconductivity semiconductor having first and second opposite sides; adiode array formed on said first side; an insulating layer, overlyingsaid second side, for storing information in the form of electriccharge; and a conductive layer, overlying said insulating layer, forcontrolling the writing and erasing of information in said insulatinglayer.
 2. The method of writing, storing, reading and erasinginformation in a diode array target, containing a diode array on oneface of a semiconductor wafer, an insulating layer overlying theopposite face of said wafer, and a transparent, conductive layeroverlying the insulating layer, comprising the steps of: selectivelyirradiating the insulating layer to induce mobile electrons therein;drawing off said mobile electrons to selectively charge said insulatinglayer by biasing said conductive layer to one polarity; inducingelectron-hole pairs in said wafer; preventing recombination of saidpairs and allowing at least one of said electrons and holes to diffuseto the diode side of said wafer; and scanning the diode side of saidwafer with an electron beam to read out said stored information.
 3. Themethod as set forth in claim 2 and further comprising the steps of:biasing said conductive layer to a second, opposite polarity; andirradiating the insulating layer to induce mobile electrons in saidinsulator for neutralizing the charge on said insulating layer.
 4. Themethod as set forth in claim 2 wherein said inducing step comprises:irradiating said target with photons of light.
 5. The method as setforth in claim 2 wherein the step of selectively irradiating comprises:selectively irradiating the insulating layer with an electron beampenetrating said conductive layer to induce mobile electrons in saidinsulating layer.
 6. An information storage system comprising asemiconductor diode array target comprising a planar semiconductorsubstrate having a diode array on one side and an insulating layercovering the opposite side; means for storing information in the form ofelectric charge in said insulating layer; means for irradiating saidopposite side of said target for inducing electron-hole pairs in saidsubstrate, said stored charge preventing recombination of said pairs toallow diffusion of at least one of said electrons and holes toward thediode array side of said target, thereby transferring the informationfrom one side of the target to the other; and means for reading saidinformation from said diode array.
 7. The information storage system asrecited in claim 6 wherein said target further comprises a transparent,conductive lAyer overlying said insulating layer and said means forstoring information comprises: a source of electron beam for irradiatingselected portions of said insulating layer; and bias means coupled tosaid transparent, conductive layer and said substrate for biasing saidconductive layer relative to said substrate.
 8. The information storagesystem as set forth in claim 6 wherein said means for reading saidinformation comprises: a source of an electron beam for irradiatingselected portions of said diode array.
 9. The information storage systemas set forth in claim 6 wherein said means for irradiating said targetcomprises a source of light.
 10. The information storage system as setforth in claim 9 wherein said light is blue.